This invention relates to peptides having antibacterial and antifungal properties. The invention also concerns the preparation of these peptides and compositions containing the same which may be used in agriculture and for human or animal therapy.
Nature provides a context in which organisms across the phylogenetic spectrum are confronted by potential microbial pathogens. In turn, natural selection provides a corresponding requirement for rapid and effective molecular stratagems of host defense against unfavorable microbial infection. Antimicrobial peptides represent a key result of this co-evolutionary relationship. While higher organisms have evolved complex and adaptive immune systems, virtually all organisms rely upon primary innate immune mechanisms that are rapidly deployed to ward off microbial invasion. Discoveries over the last decade indicate that antimicrobial peptides elaborated by essentially all organisms play integral roles in these innate mechanisms of antimicrobial host defense.
Antimicrobial peptides may be generally categorized as those with or without disulfide bridges. Those that contain disulfides commonly adopt β-sheet structures, while those lacking cysteine crosslinkages often exhibit α-helical conformation. Antimicrobial peptides from both classes have a number of conserved features that likely contribute to their toxicity to microorganisms, including: 1) small size, typically ranging from 12-50 amino acids; 2) cationicity, with net charges ranging from +2 to +7 at pH 7; and 3) amphipathic stereogeometry conferring relatively polarized hydrophilic and hydrophobic facets (Yeaman and Yount, Pharmacol. Rev. 55:27 (2003)). The limited size of these polypeptides places restrictions on the structural repertoire available to meet these requirements. Despite these limitations, as a group antimicrobial peptides display a high degree of variability at non-conserved sites, with amino acid substitution rates on the order of those associated with positive selection (A. L. Hughes, Cell. Mol. Life Sci. 56:94 (1999)). These observations are consistent with the hypothesis that co-evolutionary selective pressures drive host-pathogen interactions (M. J. Blaser, N. Engl. J. Med. 346:2083 (2002)).
Amino acid sequence motifs have previously been identified within certain antimicrobial peptide subclasses (eg., the cysteine array in certain mammalian defensins; White et al., Curr. Opin. Struct. Biol. 5:521 (1995)). Yet, comparatively little is known about more comprehensive relationships uniting all antimicrobial peptides. Conventional sequence analyses performed have yielded limited sequence conservation, and no universal structural homology has been identified amongst antimicrobial peptides. If present, such a consensus motif across the diverse families of antimicrobial peptides would provide insights into the mechanism of action of these molecules, yield information on the evolutionary origin of these sequences, and allow prediction of antimicrobial activity in molecules recognized to have other functions.
The ability of certain bacteria such as M. tuberculosis and S. aureus among others, to develop resistance to antibiotics represents a major challenge in the treatment of infectious disease. Unfortunately, relatively few new antibiotic drugs have reached the market in recent years. Methods for administering new classes of antibiotics might provide a new scientific weapon in the war against bacterial infections.
There are only a handful of antifungal drugs known for the treatment of mammals. In fact, there were only ten FDA approved antifungal drugs available in 2000 for the treatment of systemic fungal infections. There are three important classes of fungal drugs for the treatment of systemic infections: polyenes, pyrimidines, and azoles. The FDA has also approved certain drugs belonging to other classes for topical treatment of fungal infections. Certain traditional antifungal drugs may have a significant toxicity, and certain antifungal drugs available for use in treatment have a limited spectrum of activity. Still further, certain antifungal drugs among the azoles can have interactions with coadministered drugs, which can result in adverse clinical consequences. As with the antibiotics, certain fungi have developed resistance to specific antifungal drugs. Patients with compromised immune systems (e.g., AIDS) patients have in some cases had prolonged exposure to fluconazole for both prophylactic and therapeutic purposes. In 2000, increased use of the drug fluconazole correlated with the isolation of increasing numbers of resistant infectious fungi among AIDS patients. Methods of using a new class of antifungal drugs could make new treatments for fungal infections possible.
Invasive mycoses are very serious infections caused by fungi found in nature and which become pathogenic in immunocompromised persons. Immunosuppression may be the result of various causes: corticotherapy, chemotherapy, transplants, HIV infection. Opportunistic fungal infections currently account for a high mortality rate in man. They may be caused by yeasts, mainly of Candida type, or filamentous fungi, chiefly of Aspergillus type. In immunosuppressed patients, failure of antifungal treatment is frequently observed on account of its toxicity, for example, treatment with Amphotericin B, or the onset of resistant fungi, for example resistance of Candida albicans to nitrogen derivatives. It is, therefore, vital to develop new antifungal medicinal products derived from innovative molecules. In this context, antimicrobial peptides offer an attractive alternative.
Antimicrobial peptides are ubiquitous in nature and play an important role in the innate immune system of many species. Antimicrobial peptides are diverse in structure, function, and specificity. A number of antimicrobial peptides occur naturally as “host-defense” compounds in humans, other mammals, amphibians, plants and insects, as well as in bacteria themselves. Synthetic antimicrobial peptides have also been described, including highly amphipathic peptides whose amino acid sequences are related to or derived from the sequences of various viral membrane proteins.
The significant advantage of peptide antimicrobials resides in the global mechanism of their anti-microbial action; because peptides have an inherent capacity to bind and penetrate biological membranes, these compounds act by physically disrupting cellular membranes, usually causing membrane lysis and eventually cell death. Organisms such as bacteria have little ability to combat this physical mechanism and acquire resistance.
Thus, there exists a need for employing multidimensional proteomic techniques to determine structural commonalities amongst peptides elaborated in phylogenetically diverse organisms—microbial to human—and explore the potential convergence of structural paradigms in these molecules. The present invention satisfies this need and provides related advantages as well.